Migration och vegetationsupptag av radiocesium i åkermark - en långtidsserie av mätdata samt en långsiktig simuleringsmodell
Abstract
The accident in 1986 at the nuclear power station in Chernobyl, Ukraine, dispersed radioactive cesium over large areas of Europe. The half life of 137Cs is 30 years and this radionuclide also behaves like potassium, which means that it can easily be taken up by the vegetation or be adsorbed in to the structure of clays, principally illitic clays. When radiocesium decays ionizing radiation is emitted, which is harmful to humans that are exposed, either directly or indirectly through food intake. It is there-fore important to know what happens to the cesium that is dispersed into the environ-ment.
In this project a simulation model was constructed to simulate the adsorption and transport of radiocesium in the soil and plant uptake. The model was parameterized from literature values, measured values, calibration and evaluated using measured data from a field site called Skogsvallen 50 km northwest of Uppsala, where meas-urements have been taken five times (1987, 1992, 1994, 2000, 2004) since the acci-dental release of radiocesium 1986. The results regarding transport show that the model works best 1 year after deposition (Model efficiency, EF, 0.77).
The parameters regarding vegetation uptake were more difficult to explain (EF 0.46 as the best). One year after deposition the uptake in reality was large, but it was not pos-sible to include that fact in the model without a large overestimation later on. Some of the measured activity in the upper cm should be classified as activity in the vegetation instead of the soil, because of the root mat, but it is very difficult to separate the many fine roots from the soil. The dense roots at the soil surface could also contribute to less adsorption during the first few years after deposition.
For the whole model, EF was calculated as 0.56. To improve the simulation model, more detailed information about the relation between Kd, clay content and organic matter in the soil profile is desirable.
The investigation of the cesium activity in the soil profile at Skogsvallen 2004 shows that 18 years after the accident, 80% of the total activity still remains in the upper 5 cm of the soil. This means that adsorption to clay minerals is strong. Even if this lim-its vegetation uptake, it is still important to consider how strongly contaminated soils should be used in the future.